1. Field of the Disclosure
The disclosure relates to polyazacrown-based materials as well as methods for their manufacture and use, for example regenerable polyazacrown polymers used to remove mercury from contaminated water.
2. Brief Description of Related Technology
Mercury released into the environment can poison people either in its released form (e.g., Hg, Hg2+) or after biotransformation into even more poisonous derivatives, for example methylmercury (MeHg+) and dimethylmercury (Me2Hg). Mercury and its derivatives tend to concentrate in living organisms which often lack a mechanism of mercury removal (e.g., metabolism, excretion). Such accumulated mercury can then be introduced into the human food chain. In particular, mercury cations (Hg2+) accumulate in aquatic life forms as methylmercury, for example being present in algae and zooplankton at levels of about 30 ng/g to about 50 ng/g, in forage fish at a level of about 500 ng/g, and in game fish (i.e., which can be consumed by humans) at a level of about 1300 ng/g. In humans, mercury mainly concentrates in kidneys and neural cells, thus decreasing the IQ of a poisoned individual and causing irreversible damage to children. For example, it is estimated that between 300,000 to 600,000 US children have a blood mercury content at levels of at least 5.8 μg/L (Trasande et al., Environmental Health Perspective, 2005, 113(5), p. 590-596).
Generating about 48 tons/year of mercury, coal-burning power plants account for over 40 percent of all human-based mercury emissions in the United States, which makes them the major artificial source of mercury in the environment. Given increasing energy costs, the search for alternative fuels (e.g., relative to crude oil), and the abundance of domestic coal reserves, coal liquefaction processes used to produce synthetic fuels may become attractive energy alternatives. In this case, however, the potential environmental contamination due to mercury emissions can be expected to increase.
Economic losses in the United States resulting from mercury poisoning are estimated to be about $8.7 B/year, with about $1.3 B/year being attributable to coal-originated mercury emissions. In response to these losses, the US EPA issued in 2005 the Mercury Air Mercury rule that aims to reduce mercury emissions from coal power plants to 38 tons/year by 2010 and to 15 tons/year by 2018.
A number of methods for capturing mercury and its derivatives have been developed. Such methods include microbial demercuryzation (Leonhaeuser et al., Engineering in Life Sciences, 2006, 6(2), p. 139-148), mercury absorption on powdered activated carbon (PAC) which may be sulfur-impregnated for higher efficiency (Vidic, Environmental Separation of Heavy Metals, 2002, p. 15-44), precipitation of mercury sulfide (Ross, U.S. EPA Report EPA-670/2-73-080, 1973) or thiolates (Atwood et al., “Recent Developments in Mercury Science,” in Structure and Bonding, 2006, 120, p. 163-182) from aqueous solutions, absorption on thiol-modified mesoporous materials (Liu et al., Advanced Materials, 1998, 10(2), p. 161-165; Liu et al., Chemical Engineering & Technology, 1998, 21(1), p. 97-100), capture with non-selective synthetic polymers (e.g., ion-exchange resins (Calmon, Ion Exch. Pollut. Control, 1979, 1, p. 201-206), non-specific complexing materials (Michelsen et al., U.S. NTIS Report PB-244890, 1975), shredded tire rubber (Russell, U.S. NTIS Report DP-1395, 1975), or natural polymers (e.g., animal's wool (Tratnyek et al., U.S. NTIS Report t Report PB-211128, 1972)). Complex techniques utilizing more than one separation method or process have been considered as well (Buckley et al., Canadian Report AECL-10174, 1990; Nichols et al., U.S. NTIS Report OWRT-C-200009-R(2410)(1), OWRT-RU-84/6; Order No. PB84-228212, 1983; Okamoto et al., U.S. NTIS Report PB-249848, 1975; Wing et al., U.S. Agr. Res. Serv., West. Reg., Report ARS-W-19, 1974, p. 26-31).
However, the existing techniques do not allow for highly selective binding of mercury by air-stable, regenerable materials exhibiting a high capacity for mercury. For example, microbial demercurization leads to the reduction of mercury ions and formation of mercury atoms that either agglomerate into small mercury droplets or evaporate into the atmosphere. Thus, the mercury is not removed from the environment in a concentrated form. Ion-exchange resins are not selective, which leads to mercury displacement by other, more environmentally abundant metals (e.g., copper), that limits the capacity of such resins. The soft nature of mercury cations leads to their tendency to form complexes with polyamines and thiols, which may be used for mercury capture and removal. Similar to the process taking place in the ion-exchange resins, however, the typically much more environmentally abundant copper forms even more stable complexes with polyamines and catalyzes the oxidation of thiols into disulfides, thus reducing the binding ability of these compounds. Sorbents, like finely divided tire rubber, sulfur-doped activated carbon, and animal's wool can absorb mercury and decrease its concentration in water by a factor of about 100 to about 1000. However, the problem of sorbent regeneration has not been solved, and the only proposed way to deal with such materials after mercury absorption is combustion, which leads to the re-release of mercury into the environment.
Thus, there exists a need for more effective and versatile materials and processes for capturing mercury and removing it from the environment. Preferably, the new materials will (1) have a high capacity for mercury removal, (2) be highly selective for mercury (i.e., relative to other, less hazardous and potentially more abundant environmental contaminants), (3) be air-stable, and (4) be amenable to re-use via a suitable regeneration process.